This project investigates the interaction of developmental and regenerative processes in restoring the specificity of neuromuscular connectivity following transection of motor axons in bullfrog tadpoles. By specificity of connectivity, I mean the consistent relation found in the adult frog between motoneuron position within the lateral motor column (LMC) and the embryonic origin of the target muscle. Specificity of connectivity will be mapped both during normal development of hindlimb innervation and after regeneration of previously transected motor axons in developing tadpoles. In the tadpole and adult frog, as in avian and mammalian species, motoneurons innervating muscles derived from the embryonic dorsal premuscle mass lie in different regions of the LMC than those innervating muscles derived from the ventral premuscle mass. Specificity of connectivity will be assessed by applying small amounts of the retro-gradely transported histochemical marker, horseradish peroxidase, to particular hindlimb regions and determining the location of labeled motoneurons within the LMC. Functional recovery could occur despite lack of regenerative specificity if synaptic inputs to regenerated motoneurons were remodeled in a manner appropriate to the muscles they now innervated. This possibility will be examined by recording from hindlimb motor nerves during activation of the neuronal circuitry mediating hindlimb participation in swimming. Evidence consistent with synaptic remodeling would be obtained if neural activity in regenerated hindlimb motor nerves were functionally appropriate despite inappropriate connections formed by regenerated motor nerves. Regeneration of previously transected motor axons will be distinguished from formation of new neuromuscular connections in young animals by the use of a sequential double label technique. Horseradish peroxidase will be applied to hindlimb before transection of motor axons. After regeneration, tritiated, enzymatically inactive horseradish peroxidase will be applied to the same hindlimb region. The presence of motoneurons containing both chromogenic and radioactive horseradish peroxidase would demonstrate true regeneration.